Incidence of major depression: prediction from subthreshold categories in the Stirling County Study

Validity of the newly-proposed categories of Minor Depression (MinD) and Subsyndromal Depression (SSD) would be strengthened if they were found to be related to the incidence of Major Depressive Episode (MDE). In this report, the subsequent incidence of MDE was assessed in terms of baseline evidence about the two subthreshold categories and Dysthymic Disorder (DysD).

The Diagnostic Interview Schedule was used to interview 489 subjects twice between 1991 and 1995 as part of the Stirling County Study. Life table methods were used to calculate incidence rates and log linear analysis was employed for statistical assessment.

The average annual incidence of MDE was 10 per 1000. The rates associated with the baseline categories were 210 per 1000 for DysD, 21 for MinD; 13 for SSD; and four for the remainder of subjects who constituted the reference group. Only the incidence related to DysD was significantly higher than that for the reference group. Comparing Time 1 and Time 2, one-third of the subjects gained or lost symptoms and the comorbidity of MDE and DysD increased.

The study population was, on average, 63 years of age. Older age and the small size of the cohort may have influenced the findings. Another limitation may relate to the use of lifetime rather than current symptomatology.

DysD is strongly associated with MDE suggesting that the chronic and episodic forms of unipolar depression constitute one disorder. The other subthreshold categories should be further investigated in terms of prodromal qualities. The persistence of dysphoria, as in DysD, may be a more important feature of the depression prodrome than the number of symptoms.